This invention relates generally to air conditioning evaporators and, more particularly, to plate heat exchangers with two-phase refrigerant flow distribution.
In the cooling phase of a refrigeration system the heat exchanger referred to as an evaporator receives liquid refrigerant by way of an expansion valve, with the expanding refrigerant then tending to cool the liquid being separately circulated through the evaporator. The fluid to be cooled carries the heat load which the air conditioner is designed to cool, with the evaporator then transferring heat from the heat load to the liquid refrigerant.
One type of heat exchanger used as an evaporator is a brazed plate heat exchanger wherein a plurality of parallel plates define passages, and provision is made for the flow of refrigerant and water in alternate passages so as to effect a heat exchange relationship therebetween. In such a heat exchanger, refrigerant is distributed to alternate channels by way of a manifold extending across on end of the channels. A problem that occurs is that a two-phase refrigerant flow entering the manifold from an expansion valve tends to flow unevenly into the individual channels as it proceeds across the length of the manifold. This is particularly true for larger systems i.e., for example, greater than an 80 ton air conditioner. That is, as the refrigerant flow moves along the manifold, flow rate depletion causes two-phase flow pattern to change, resulting in a maldistribution to the individual channels.
One approach to solve this problem has been to form an orifice at the inlet of each of the refrigerant channels to thereby create a pressure drop and improve the quality of vapor passing into the channels. However, the problem of maldistribution still exists and limits the use of brazed plate heat exchangers to around 100 ton capacity with refrigerants such as R-134a.
Another common approach to solving the problem is to use a liquid-vapor separator to separate the liquid and vapor phases coming from the expansion valve. This can be accomplished by either an internal or external liquid-vapor separator. However, in either case such an addition represents a substantial increase in cost, weight and manufacturing complexity.
It is therefore an object of the present invention to provide an improved method and apparatus for refrigerant distribution in a brazed plate heat exchanger.
Another object of the present invention is the provision for effectively distributing two-phase refrigerant in a brazed plate heat exchanger.
Yet another object of the present invention is the provision for an improved method and apparatus for distributing two-phase flow in a uniform manner to a plurality of channels in a plate heat exchanger.
Still another object of the present invention is the provision for a brazed plate heat exchanger that is economical to manufacture and effective and efficient in use.
These objects and other features and advantages become readily apparent upon reference to the following descriptions when taken in conjunction with appended drawings.
Briefly, in accordance with one aspect of the invention, a manifold which receives two-phase refrigerant from the expansion valve, is provided with a nozzle which provides a pressure drop and an increase in velocity to propel the two-phase refrigerant flow into the manifold. In this way, the nozzle provides a motive force for a non-stratified flow of the two-phase refrigerant mixture through the manifold to thereby ensure a uniform distribution to the individual channels that are fluidly interconnected to the manifold.
By yet another aspect of the invention, the manifold is a two-pass structure interconnected by a return bend, with the first pass having openings that are fluidly connected to refrigerant channels of the plate heat exchanger, and the second pass is simply provided to return the flow from the return bend to the nozzle at the other, upstream, end of the first pass. The structure of the manifold thus provides a closed circuit such that the refrigerant makes a complete cycle through the manifold to return to the nozzle.